The invention relates to a superconducting short circuit current limiter for an alternating current network, comprising a closed iron core with a primary winding which is part of an electric power supply line to be protected and a short-circuited superconducting secondary winding disposed on the iron core in spaced relationship from the primary winding whereby the current in the secondary winding cannot rise beyond a certain value when the nominal current flow in the primary winding is exceeded.
The control of short circuits in electric power supply nets becomes increasingly expensive with the expansion and interfacing of the power supply networks. With the measures commonly in use at this point, such as adapted short circuit voltages of the transformers and sufficient power line impedances, rapid circuit breakers with tripping currents of sometimes far above 50 kA and inserted short circuit limiting throttles, the electric power supply nets cannot be protected adequately. This is especially true if, with continuing deregulation, the net configurations and operating conditions can no longer be freely selected on the basis of the given short circuit capabilities of the system. High voltage rigidity during separation is basically not compatible with short circuit limiting impedances. Consequently, new types of short circuit protection methods or needed. Conventionally, for example, the current has been limited by the First-Zero-Interrupter (FZ1) technique. Its essential disadvantage however is the unlimited occurrence of possibly high-impulse short circuit amplitudes which subjects the equipment components to high mechanical forces and possible failure.
It is desirable to limit or eliminate already the impulse short circuit current. This requires a current limiter which acts without delay when a certain current level is exceeded. Such a rapidly acting over current protection is especially necessary if the net includes superconductive building components.
Current limiters with such a property are the superconductive fault current limiters (SCFCL). For such applications transformers have been proposed whose primary coils are connected at a predetermined location of a network to be protected. They have a superconducting secondary coil, for example, in the form of a cylinder which is short circuited. Both coils are disposed on an iron core and are therefore magnetically highly coupled with each other up to a point at which the iron core is saturated.
The operation of such a superconducting short circuit current limiter is based on the fact that, when the current in the primary coil exceeds the nominal current I.sub.N an impedance Z.sub.(t) is built up in the short circuit as the current increases above the response current level I.sub.T. This impedance Z.sub.(t) grows rapidly from zero to an end value and limits the current to an acceptable current level of: EQU I.sub.lim =U/Z.sub.lim
In a first approximation, U is the net voltage which is considered to be constant. (In a low resistance grounded rotating current net each phase needs to be provided with such a short circuit limiter for an effective protection).
J. Acero et al., describes on pages 1071 to 1074 of "Current Limiter Based on Melt Processed YBCO Bulk Superconductors" published in IEEE Trans. on Appl. Superconductivity, Vol. 5, No. 2, June 1995, the design and manufacture of a superconducting current limiter.
In the same publication on pages 1059-1064, W. Paul et al., describe a test of a symmetrically constructed high--Tc superconducting current limiter with a three-leg yoke.
DE-OS 195 24 579 discloses as current limiter a transformer wherein fault currents to be limited flow through the primary coil and which includes a superconductive ring as a secondary coil. The secondary coil is cooled in a cooling container below the critical temperature of the ring material. The ring is formed by a thin annular layer on a carrier cylinder and forms therewith a compound body.
Basically, such a superconductive short circuit limiter is an axial transformer which acts as a non-linear current-dependent impedance.
It operates as follows:
The primary coil of the transformer is installed in the power line to the be protected. It is at line voltage. The power line current is, consequently, the primary current I.sub.p of the transformer. During normal operation, that is, from no load up to normal current flow, the short-circuited superconductive secondary coil is always counter-excited up to the full primary current flow compensation. By design, the secondary coil is a short circuited coil or a short circuit ring or cylinder. Particularly, a cylinder is quite suitable for the manufacture from high temperature superconducting materials.
The ideal transformer generates no voltage drop at its primary coil since the flux in the iron is zero. There is no stray flux or it is negligible. When the response level current I.sub.T is exceeded also the critical current flow I.sub.skrit, that is the limit of the superconducting current flow capacity of the superconductor is reached. The secondary current flow cannot further increase from here on even with good cooling. As a result, a magnetic field flux is generated in the iron core of the transformer dependent on the power line short circuit current flow which builds up in the primary coil a backlash voltage which limits the short circuit current. At the same time, in the secondary coil a voltage is induced which, together with the residual current still flowing in the superconducting coil, generates heat losses heating the superconductor.
For limiting unacceptably high switching over-voltages in the net, a normally conductive tertiary coil is wound onto the iron core.
With the usual size dimensioning of such an axial transformer the line voltage and the normal current value were the decisive factors. Iron saturation was to be avoided and the cross-sections were sized for assymetric short circuits, that is, the iron core cross-section was then twice that of a corresponding power transformer. This however results in high iron weights.
It is the object of the present invention to provide a superconducting short circuit limiter which fulfills the following requirements: It should provide for:
a negligible impedance and negligible operational losses during normal operation; PA1 well defined impedance and acceptable losses while a short circuit occurs; PA1 a time dependent control of the impedance build up z(1) such that the impulse short circuit current remains securely limited but, on the other hand, no unacceptably high over-voltages will be initiated, PA1 rapid availability at the end of the short circuit, PA1 a compact design having the least possible weight, PA1 high operational safety during long term operation, PA1 automatic operation.